Chromatography is a known method of analyzing a sample comprised of one or more components to qualitatively determine the identity of the sample components as well as quantitatively determine the concentration of the components.
A typical chromatographic apparatus includes: a sample introduction system having an injection port, into which the sample is injected and mixed with an inert fluid at high temperature; a column through which the various dissolved components of the sample will travel at a rate related to the characteristics of the specific components; and a detector for measuring the presence of each component.
Analytical chromatography involves a series of steps: sample collection, sample preparation, sample introduction into the chromatographic system, chromatographic separation into individual components, detection of those components, and data acquisition and reduction. For each step, the analyst must make appropriate choices among accepted procedures and available instrumentation. Improper selection or use of the sample introduction system can dramatically limit the performance of the system and, therefore, the ultimate performance of the analytical method. Because of the great variety of columns and the diversity of samples that can be analyzed with modern chromatography, several sample introduction systems and related injection modes are known; no single system can best satisfy all analytical requirements.
However, a common function of the sample introduction system is to provide accurate, reproducible, and predictable introduction of sample into the column. Usually the sample introduction system includes a device known as a sample inlet, whereby a quantity of sample in a liquid form is injected using a syringe. There are other sample introduction devices that introduce samples into the chromatographic column when syringe injection is inappropriate (for example, with solid samples): a gas or liquid sampling valve, head space autosampler, thermal desorber, purge and trap sampler, or pyrolyzer.
Sample inlets are usually divided into two major categories: packed column inlets and capillary column inlets. The types of capillary column inlets are known to include: capillary direct, split/splitless, programmed temperature vaporizing (PTV), and cool on-column direct. Nearly all capillary inlets are vaporizing, including on-column direct injection, except for cool on-column injection, which deposits condensed sample directly into the column.
A useful technique for introducing a liquid sample into a chromatographic system is by injection into a hot inlet wherein it is quickly vaporized (flash vaporization). The benefits of flash vaporization includes the transfer of the liquid sample to a gas and a quick transfer of a sample into the column. However, typical problems associated with vaporizing inlets include band broadening, needle discrimination, inlet discrimination, and sample decomposition. Sample decomposition is indicated by lost or misshapen peaks, or by the generation of unwanted peaks. Decomposition is exacerbated by high inlet temperatures, long residence time of the sample in the inlet, and chemical activity of the sample with the inlet. These problems are exacerbated by the use of high inlet temperatures and low boiling point solvents.
Accordingly, an inlet liner may be positioned in the inlet to reduce chemical activity between the sample and the inlet. The selection and construction of the inlet liner has a direct effect on the success of an analysis, and each type of inlet is designed to function best with a certain type of liner. For example, splitless inlets may require straight liners with no packing, whereas liners constructed for other techniques, such as programmed temperature vaporization, require baffled or packed liners to retain a liquid sample during cold sample introduction.
The inlet liner and any packing material therein is expected to be non-reactive with respect to the compounds and solvents that may be present in an injected sample. One conventional approach is to chemically deactivate the liner and its contents. Deactivation reagents such as hexamethyldisilazane (HMDS), dimethyldichlorosilane (DMDS), and polymethylhydrosiloxane (PMHS) are typical examples. In another approach, a stationary phase coating may be applied to a particulate (such as Dexsil-300 on Chromosorb 750) to achieve a deactivated surface. Such a procedure may only be effective for a limited series of analytical runs, after which the liner must be cleaned and again be deactivated. Thus, conventional approaches can be unsatisfactory, as will now be described.
Note that any sample decomposition will undesirably degrade the minimum detectable level (MDL) of the chromatograph. Low detection levels are important in environmental, pharmaceutical, food analysis, and other gas chromatography applications. Improvements in sample handling, sample injection techniques, and detectors have all contributed to the ability to measure compounds at decreasing levels.
Sample decomposition is especially undesirable in large volume injection (LVI), which is an important technique for lowering the minimum detection level. In LVI, a large volume of sample is injected. The bulk of the solvent is evaporated before the transfer of the sample to the analytical column is initiated. Large volume injection is especially useful in trace analysis to improve analyte detectability, for analysis of, e.g., pesticides and pollutants. Very often it can replace an off-line evaporation step carried out to concentrate a diluted sample extract.
New inlets and injection techniques supporting large volume injection (LVI) have been developed in recent years. Accordingly, there are two primary techniques used to eliminate solvent: 1) via a programmable temperature vaporizer (PTV) inlet; and 2) via a cool on-column injection with solvent vapor exit (COC-SVE). Programmed temperature vaporizing injectors (PTV) have been shown to be well-suited for large volume sample introduction in capillary gas chromatography (cf., e.g., Wilson et al., "Large Volume Injection for Gas Chromatography Using a PTV Inlet", Application Note 228-374, Hewlett-Packard Company, March 1997). LVI with PTV is ideal for trace analysis of later-eluting solutes (i.e., solvents having boiling points approximately 100.degree. C. higher than the solvent) and for dirty samples.
Note that the typical injection volume for capillary column analysis is 0.5 to 2 .mu.l. The Hewlett-Packard HP 5890 and HP 6890 series gas chromatographs allow approximately two times the normal injection volume (up to 5 .mu.l, depending on the solvent) using "pulsed" splitless injection. Injecting still larger volumes with standard techniques can lead to contamination of the system, irreproducible results, and loss of sample. In contrast, the typical injection volumes employed in a large volume, solvent elimination PTV inlet are 25 to 100 .mu.l; even up to 1 ml have been demonstrated. Multiple injections can be used with the PTV inlet when even larger volumes are required.
Hence, for large volume injections, the PTV inlet is often used in a "solvent vent" or "solvent elimination" mode. Sample is introduced into the inlet with the inlet temperature near the boiling point of the solvent and with a relatively high split ratio. The solvent (and low-boiling solutes) is vented while the higher boiling solutes (more than about 100.degree. C. above the solvent boiling point) remain and are concentrated in the inlet. After a predetermined period, the split vent is closed and the inlet temperature is increased to transfer the solutes and any residual solvent to a column for separation. Because the sample is evaporated from the inlet, nonvolatile sample components and degradation products remain behind in the inlet, minimizing column contamination. Thus, the PTV inlet is chosen more often than cool-on-column or split/splitless inlet for receiving dirty samples. Hence the PTV inlet is advantageous for cold split or splitless applications and avoids most of the problems associated with hot inlets such as sample discrimination, liner overload, and sample decomposition.
Two injection techniques for large volume injection (LVI) are known: controlled speed injections and multiple injections. In controlled speed injections, the sample injection rate is matched to the solvent vaporization rate such that the liner is not overfilled with liquid during the sample injection. Hence the inlet liner capacity is not a limiting factor, but the injection rate must be slowed to an appropriate speed.
In multiple injections, an aliquot of sample that does not exceed the liquid capacity of the liner is introduced to the inlet. The solvent is allowed to evaporate before the next aliquot is injected. This approach has several advantages over controlled speed injection. However, at least one of the disadvantages of multiple injections is the single shot liner capacity: for a typical liner (e.g., a multibaffle liner) the liner capacity is approximately 5 .mu.l. Thus, to make a 100 .mu.l injection, 20 replicate injections must be made. A larger liner can reduce the number of replicate injections and minimizes adverse effects such as vial cap septum coring and contamination, but such liner construction suffers from other disadvantages, such as its cost, size, etc.
Liner capacity may be increased by packing the liner bore with a support material in order to provide a physical support for retention of the liquid sample in the liner bore after injection. Several materials are known; the typical choice is quartz or glass wool, which provides good permeability as well as a large surface area for holding solvent. A typical liner packed with glass wool has an injection capacity of at least 35 .mu.l. Another approach is to pack the liner with glass beads or particulates used for packing chromatographic columns, such as Tenax TA or Dexsil-300 coated on Chromosorb 750 (commercially available from Chrompack, Bergen op Zoom, The Netherlands). The small particles (35-200 mesh size) are porous and offer large surface area for holding solvent.
Mol et al., in "Large Volume Injection in Capillary GC Using PTV Injectors", J. High Resol. Chromatogr., 18, 124-128, (February 1995), describe use of a variety of packing materials, including silanized glass wool, which is often used and which affords reliable results for thermostable compounds such as polycyclic aromatic hydrocarbons and polychlorobiphenyls. However, the authors conclude that the use of silanized glass wool as packing material in a large volume sample introduction with a PTV injector causes decomposition of thermolabile or polar compounds because the glass wool exhibits an undesirable interaction between the sample and active sites on the surface of the wool fibers. As a result there is degradation or adsorption of the analytes when present in the liner.
The authors also found certain alternatives to glass wool-packed liners that were more inert than glass wool and were better suited as packing material in large volume sample introduction for analytes covering a broad volatility and polarity range. Their preferred alternative was a PTV inlet packed with PTFE wool or a packing material composed of the above-described Dexsil-coated Chromosorb material. With regard to the packing material composed of Dexsil-coated Chromosorb material, the authors describe only their use of hexane and ethyl acetate as a sample solvent. Neither of these solvents would be expected to dissolve Dexsil; however, other solvents are capable of such dissolution. Accordingly, the Dexsil-coated packing material has limited use as it would not be suitable for use with a solvent capable of dissolving such a coating.
The other preferred packing material, i.e., PTFE wool, is described by the authors as being prepared from a PTFE rod, and is cited as having a maximum inlet temperature of 275.degree. C. The authors therefore appear to teach the use of PTFE wool only for use in inlets subject to lower temperatures; in fact, the authors recite the basis of their choice of PTFE as being its water resistance.
Accordingly, for analyses of pesticides and pollutants, wherein polar and thermally-labile compounds are so prevalent, the foregoing approaches are less than satisfactory because of temperature limitations or due to sample decomposition due to interaction of those compounds with the inlet liner and the packing material. For example, the above-described use of PTFE wool is attractive, except that a maximum temperature limit of 275.degree. C. which precludes the use of PTFE wool at higher temperatures that would be useful in certain applications. Moreover, the large surface area offered by glass or PTFE wool enhances solvent capacity but nonetheless also offers a multitude of sites for analyte adsorption or decomposition. Organophosphate pesticides such as Guthion, Acephate, and Methamidophos are prime examples of difficult compounds that may be subject to adsorption or decomposition. Lastly, it is difficult to pack a typical liner with PTFE wool in a reproducible way such that the maximum sample volumes and solvent vent times do not vary.
For the foregoing reasons, there remains a need for an improved packing material for use in all types of inlet liners including, but not limited to, split/splitless inlets and PTV inlets. Ideally, the packing should be highly inert and thermostable. The packed liner should retain the high-boiling analytes only as necessary in order to minimize the thermal stress applied to these compounds upon splitless transfer to the column. Furthermore, the packing should be compatible with common organic solvents and the inertness should not be affected by their use. The packed liner should retain a large volume of liquid sample in order to allow rapid introduction of large sample volumes without overloading the liner capacity. The packing material should be applicable to a typical liner in a reproducible way such that the maximum sample volumes and solvent vent times do not vary.
Also for the foregoing reasons, there remains a need for an improved liner for use in all types of inlet liners including, but not limited to, split/splitless inlets and PTV inlets. Ideally, the liner should be highly inert and thermostable. The liner should be compatible with common organic solvents and its inertness should not be affected by their use. The liner should be amenable to packing so as to retain a large volume of liquid sample in order to allow rapid introduction of large sample volumes without overloading the liner capacity. The liner should also be amenable to receiving packing material in a reproducible way such that the maximum sample volumes and solvent vent times do not vary.